Envlron. Scl. Technol. 1992, 26, 2116-2121
Development of a Structure-Reactivity Relationship for the Photohydrolysls of Substltuted Aromatic Halides Willie J. 0 . M. PeIJnenburg,' Karin 0. M. de Beer, Martin W. A. de Haan, Henrl A. den Hollander, Miranda H. L. Stegeman, and Hans Verboom
Laboratory for Ecotoxicology, National Institute of Public Health and Envlronmentai Protectlon, P.O. Box 1, 3720 BA Bilthoven, The Netherlands ~
~
~~
~~
~~~~~
Rates of photodegradation of 13 structurally related 1,3-di- and 1,3,5-trisubstituted halogenated benzene derivatives in dilute aqueous solution were measured in the laboratory at wavelengths ranging from 250 to 360 nm. As shown by mathematical simulations of the reaction rate constants involved, photohydrolysis of all compounds studied is the main transformation process, generally accounting for over 95% of the amount of starting material removed. Quantum yields of this process were calculated using 3-chlorophenol as a reference, taking into account both the absorption characteristics of the compounds studied and the spectral energy distribution of the light source used. In an attempt to develop a structure-reactivity relationship, measured quantum yields of photohydrolysis were correlated to a number of readily available molecular descriptors. As shown by statistical analysis, the best correlation was obtained using a combination of the following descriptors: (1)the carbon-halogen bond strength and (2) the summation of the steric factors of all substituents.
Introduction Assessing the risks to the environment associated with the production and use of chemicals requires available mathematical models that can be used to predict both the toxicity characteristics of the compounds concerned and their physical/chemical behavior in the environment. With regard to transformation processes considered to be important, quantitative structure-activity relationships (QSARs) between chemical reactivity and one or more physical or chemical properties of a xenobiotic are valuable tools for calculating rate constants within classes of compounds. Thus QSARs not only allow one to distinguish between transformation processes important in the chemical's removal but also enable the prediction of the products formed. At the moment an increasing number of QSARs are available for the description of various transformation processes, based on a number of divergent molecular descriptors. Most of these methods relate to strictly abiotic transformation processes such as neutral and alkaline hydrolysis (I), gas-phase oxidation of organic compounds by hydroxyl radicals (2),and reductive dehalogenation in anoxic systems (3). As far as photochemical transformations are concerned, up until now methods for the prediction of rates of photolysis and the nature of the products formed are lacking. This may be explained partly by the observation that upon irradiation of organic compounds a diverse number of primary and secondary photochemical reactions often occur, leading to formation of a variety of different products. Rates and selectivity of photochemical processes have been shown to depend on various additional factors, for instance, wavelength, temperature, pH, and the presence of sensitizing or quenching agents (4,5). In ad-
* Author to whom correspondence may be addressed 743129; Fax +31 30 742971. 2116
Environ. Scl. Technoi., Vol. 26, No. 11, 1992
Tel. +3130
dition, compounds can be converted indirectly by reactions with photochemically produced reactive intermediates, such as hydroxyl radicals, hydrated electrons, and singlet oxygen. Chlorobenzene and chlorophenols are groups of compounds whose photochemical behavior in aqueous solution has been studied extensively in recent years [for reviews, see Boule et al. (6,7)and references cited therein]. These compounds are important because of their toxic effects and known persistence in the environment. It was found that upon direct excitation of monosubstituted halobenzenes and a limited number of multiply substituted chlorobenzenes and chlorophenols, a specific heterolytic scission of the carbon-halogen bond takes place, leading to formation of the corresponding hydroxylated derivatives as the primary products (photohydrolysis). As proposed by Bunce et al. (8),reaction takes place from the first triplet excited state. The reaction was shown to be specific (generally accounting for over 80% of the starting material converted) for compounds having substituents at the meta position. In contrast, photocontraction of the aromatic ring was shown to be the dominant pathway for compounds bearing substituents at the ortho position, whereas a mixture of products was formed upon irradiation of para-substituted halobenzenes. It was stressed by Tissot et al. (9) that the specificity and the kinetics of the photohydrolysis do not depend on experimental conditions such as the excitation wavelength, oxygenation, the initial concentration of the starting compound, and the presence of cosolvents. For substituted phenols it was found that the reaction quantum yields for the anionic form are slightly higher than for the molecular form. In order to show that it is indeed possible to derive photochemical QSARs that can be used to predict rates of photolysis of organic chemicals, in this paper, we report on newly developed methods that are applicable to the calculation of rates of photohydrolysis of meta-substituted aromatic halides. The latter transformation reaction was selected since, as shown above, it has already been well described in literature. Thereupon it has been shown that, for the type of model compounds selected, this reaction dominantly takes place over other photochemical transformations, leading to the selective formation of hydroxylated aromatic compounds. From an environmental point of view, this reaction may be important since, upon introduction of a hydroxyl moiety, the octanol-water partition coefficient of the products formed will be lowered, making them more susceptible to further degradation and reducing their bioaccumulating potential. The Compounds selected include chlorinated as well as brominted, iodinated, and fluorinated benzene and phenol derivatives. An approach was followed in which first the concentrationtime profile of all starting, primary, and, if appropriate, secondary products was measured upon irradiation of the starting product, using a broad-spectrum lamp with known spectral energy distribution. In separate experiments the same procedure was used to assess rates of photolysis of the primary and, if appropriate, the secondary products.
0013-936X/92/0926-2116$03.00/0
0 1992 American Chemical Society
The data thus obtained enabled us to carry out a mathematical fit to the pseudo-firsborder differential equations for formation and disappearance of all compounds involved. From this fit it was found that, for all compounds studied, photohydrolysis is the dominant transformation process. In a next step, the quantum yields of photolysis of the starting compounds were calculated relative to 3chlorophenol. To this end, the so-called action spectrum of the compound under investigation was calculated, taking into account both its absorption characteristics and the emission spectrum of the lamp used (5). Finally, the quantum yields thus obtained were correlated to a limited number of generally available structural parameters, making the QSARs developed generally applicable. Theoretical Considerations Calculation of Quantum Yields. The average rate of direct photoreaction of a chemical at a certain wavelength, A, is equal to -(d[Cl/dt)dx = @ J a x (1) In this equation, is the average reaction quantum yield at this wavelength, and IaXis the average rate of light absorption at the wavelength, A. According to Zepp (5), Iaxis equal to l a x = 2.3031,k,[C] (2) In this equation, I x is the incident light intensity (phot ~ n s - c m - ~ dI )is, the average cell path length (cm), ex is the molar absorptivity (extinction coefficient) of the chemical (M-l-cm-'), and [C] is its molar concentration. Substituting eq 2 in eq 1, and taking the sum over all wavelengths for which the chemical absorbs light, yields (d[C] /dt)h = 2.3034 C]C(@Jx€x) (3) x
x
Now, the overall pseudo-first-order rate of photolysis can be defined as -(d[C]/dt) = kobs[C] -C(d[Cl/dt), (4) x
In this equation, kobs is the measured rate constant (s-'). Substituting eq 4 in eq 3 yields kobs = 2.3031C(@Jx~,) (5) h
As shown by Tissot et al. (IO),the quantum yield for photohydrolysis, @, is independent of the wavelength, so eq 5 becomes kobs = 2.3031@C(Ix~J (6) x
Since, as shown in Figure 1, only relative values are available for Ix,absolute values of 0 could not be measured directly. Indirectly these could be calculated using a reference compound for which values of @ were reported in the literature. After correction for side reactions, a @ value of 0.09 f 0.01 was measured by Boule et al. (11)for photohydrolysis of the molecular form of 3-chlorophenol at both 254 and 296 nm. We therefore included 3chlorophenol as one of the compounds to be studied. By application of eq 6 to both 3-chlorophenol (3CP) and a given compound, X, eq 7 is obtained. With this equation, the quantum yield of direct photolysis of the compound X can be calculated: ( I A d 3 C P ) ) (7) @X = @3CP(kobs,X/(C(IX€X)X)) / (koba,BCP/ x
x
Given the fact that ax is independent of the wavelength, as a function of A plotting the function (Ix~X)/(CX(Ix~X)) gives the relative reactivity of the chemical at a certain wavelength (12).
WAVELENGTH (nm)
Figure 1. Relative spectral energy distribution of the light source used in the spectral region 250-350 nm.
Experimental Section Chemicals. All chemicals used were obtained from common commercial sources. In most cases they were used as received; no evidence of significant impurities was found by UV-visible spectrometry or HPLC. Methanol was used as the solvent for preparing stock solutions of the compounds to be irradiated. Irradiations. UV spectra were recorded with the aid of a Hewlett-Packard 8451A diode-array spectrophotometer with quartz vessels having a path length of 1cm. For all compounds studied, the bandwidth used for recording the UV absorbance as a function of the wavelength was 2 nm. Generally, deionized water having a specific resistance of 1 6 1 8 MGcm-l, obtained from a MilliQ-system, was used as the solvent for recording UV absorption spectra. For compounds having a very low aqueous solubility, 10% acetonitrile (v/v) was added. All irradiations were carried out in a Rayonet RPR-208 merry-go-round photoreactor equipped with eight RUL 3000-A lamps. The relative energy distribution (as given by the supplier of the light source; The Southern New England Ultraviolet Co., Hamden, CT) in the spectral region 250-350 nm of the light source used is given in Figure 1. The lamps were cooled with air, and the reaction temperature was maintained at 20 f 2 "C. The usual working solution contained 1 X M initial concentration of the substrate in deionized water having a specific resistance of 1 6 1 8 M!d cm-l, assuring the solutions to be pristine and allowing only direct photolytical reactions. The pH of the solutions was -6, assuring that all phenols used were in their molecular form. The absorbance of all solutions was less than 0.02 at all wavelengths. Aging of the lamps used was checked by frequently irradiating an aqueous solution M p-nitroacetophenone and 2.47 X M of 1.54 X pyridine. This actinometer was chosen since it has been shown to absorb light moderately and uniformly in the spectral region where the emission of the light source is at a maximum (290-330 nm) (13),thus ensuring maximum sensitivity. Over the duration of the experiments, reactivity of this actinometer did not differ by more than 2%. Unless stated otherwise, all samples were irradiated using a 100-mL stoppered quartz test tube. Periodically throughout the duration of the experiment, dependent on the reactivity of the compound under investigation, 2-mL aliquots were withdrawn for analysis. In general, samples were irradiated during at least 2 half-lives. Analysis. The concentrations of unreacted substrate, primary, and, if appropriate, secondary products were determined by HPLC with UV absorbance detection. A Chromsep Vydac 201 TPB 5-pm 10 cm X 3 mm analytical column and a precolumn of the same material were used. Envlron. Sci. Technol., Vol. 26, No. 11, 1992
2117
MEASURED 1 .o
1.0
c
6
p 0.8 2
OSS 0.6
CI-3-Br-BENZENE
z
$ 0.6 0 z 0
0 0.4
0.4
>
w
-
I-
0
3
CALCULATED
z
z
i
n
3-CI-PHENOL
0.2
2
5 D
0:
40
SO
120
160
-
0.2 -
3-CI-PHENOL
c
RESORCINOL
-
200
TIME (MINUTES)
TIME (MINUTES)
Figure 2. calculated and measured concentration-time profiles for photolysis of 1-chioro-3-bromobenzene in aqueous solution.
The eluant consisted of a mixture of acetonitrile and water, with varying composition according to the compound to be analyzed. UV absorbance was monitored at the A,, of each compound. Kinetics and Statistical Analyses. The pseudofirst-order disappearance rate constant, kobs,for all substrates was determined from the slope of the line fitted by least-squares regression analysis after inspection of In C,/C, versus time plots. 1.2 values were taken as a measure of the goodness of fit. The Student's t distribution table given by Morrison (14) was used to calculate the 90% confidence intervals. These intervals are defined as being 2SE, with SE being the standard error calculated (14). Results and Discussion Kinetics. In all cases the photolysis was pseudo first order in the halogenated compound. The measured pseudo-first-order reaction rate constants, kobs,the halflives calculated from these, tl,2,the calculated r2 values, and the number of data points used are given in Table I. Table I also contains the 90% confidence intervals for both kobsand tlIz. Reaction rate constants obtained upon duplication of the whole photolytical procedure for a limited number of compounds were shown to be well within the 90% confidence intervals. All results reported in Table I were obtained using quartz test tubes. Products. For all halogenated compounds studied, photohydrolysis was the dominant transformation process, leading to formation of the corresponding hydroxylated derivatives. Dependent on the presence of halogen atoms in the primary photoproducts, these produds in their turn were photohydrolyzed, finally yielding compounds in which all halogen atoms are replaced by hydroxyl groups. For compounds having different halogen substituents solely, dehalogenation of the compounds for which the carbonhalogen bond strength is the lowest was observed. Thus, 3for example, in the case of l-chloro-3-bromobenzene, chlorophenol was the sole primary photoproduct formed; no 3-bromophenol could be detected. Calculated concentration-time profiles for both 1-chloro-3-bromobenzene and 3-chlorophenol,based upon separately measured rates of degradation, fully support this observation (see below). In order to show that photohydrolysis of all halogenated compounds studied is the dominant removal process, concentration-time profiles were calculated for all compounds involved. By use of the photolytical rate constants measured separately (see Table I), a fit to the analytical solutions of the pseudo-first-order differential equations for the disappearance of the starting compound and the formation and disappearance of the primary product and, if appropriate, the secondary product was carried out. 2118
Envlron. Sci. Technol., Vol. 26, No. 11, 1992
Table I. Measured Pseudo-First-Order Reaction Rate Constants, kobs,for Direct Photolysis of Substituted Benzene Derivatives in Aqueous Solution, Half-Lives, t I/z, Calculated from These, Calculated Regression Coefficients, r2,Using Least-Squares Regression Analysis, and Number of Data Points, n , Used" compound
kabs (min-')
t I l 2 (min)
r2
n
3-iodophenol 3-bromophenol 3,5-dichlorophenol 3-chlorophenol 3-fluorophenol 1-chloro-3-iodobenzene 1-bromo-3-fluorobenzene 1,3-dihydroxybenzene 1,3-dibromobenzene 1,3-dichlorobenzene 1,3,5-tribromobenzene 1-chloro-3-bromobenzene 1,3,5-trichlorobenzene
0.124 f 0.002 0.104 f 0.001 0.063 f 0.001 0.048 f 0.001
5.6 f 0.1 6.7 f 0.1 11.0 f 0.2 14.5 f 0.3 18.2 f 0.4 26.8 0.8
1.000
8
0.038 f 0.001 0.026 f 0.001 0.016 f 0.000 0.014 f 0.000 0.009 f 0.000 0.008 f 0.001 0.005 f 0.001 0.005 f 0.000
0.003 f 0.000
1.000 12
0.999 0.999 0.999 0.998 44.0 f 0.9 0.999 50.7 f 1.0 0.999 76.8 f 1.4 0.999 92.3 f 6.4 0.994 140.5 f 21.8 0.978 152.5 f 4.1 0.998 205.5 f 7.5 0.993
*
12
6 9 9 9 15 14 11 11
13 22
OThe 90% confidence intervals for both kobs and tl12are also given. All data were obtained using quartz reaction tubes.
Finally, the resulting concentration-time profiles were compared to the measured profiles. As an illustration, the calculated and measured concentration-time profiles for photolysis of 1-chloro-3-bromobenzeneare given in Figure 2. As can be seen from this figure, the calculated concentration-time profile, taking only initial formation of 3-chlorophenol into account, corresponds well with the measured one. From the summed concentrations of starting products and photoproducts it may be deduced that, for l-chloro-3-bromobenzene, photohydrolysis is the main transformation process, accounting for over 95% of the starting product photolyzed. The same observation was also made for most of the other halogenated compounds studied, the only exception being 1-chloro-3-iodobenzene. As shown in Figure 3, for this compound, the calculated concentrations of both 3chlorophenol and 1,3-dihydroxybenzene (resorcinol) are significantly higher than the concentrations measured. Upon irradiation of both products in a 10* M aqueous solution of sodium iodide, an iodide concentration equal to that upon 10% transformation of 1-chloro-3-iodobenzene, it was found that rates of photolysis for both compounds were greatly enhanced by the presence of sodium iodide; no effect was observed on the nature of the products formed. For 3-chlorophenol, the rate of photolysis in the presence of sodium iodide was 0.065 min-l, as compared to a value of 0.048 m i d reported in Table I; for resorcinol, these values were 0.076 and 0.014 min-l, respectively, whereas the presence of sodium iodide had no
CALCULATED
MEASU RED i n
1.0 1-CI-3-I-BENZENE
r,
2
0
$
0.8
F
50
5
0.6 0.4
2
5 W
0.2
a 0
40
Flgure
120
60
TIME (MINUTES)
160
200
TIME (MINUTES)
Calculated and measured concentratlon-time profiles for photolysls of 1-chloro-34odobenzene In aqueous solution. 0.14 3-CI-PHENOL +
0.12
.
3-CI-PHENOL
1,3,5-trlCI-BENZENE
250
270
WAVELENGTH (nm)
Figure
1,3,5-trlCI-BENZENE
+
290
310
330
350
WAVELENGTH (nm)
Molar absorptlvlty and relative reactivity as a function of wavelength for 3-chlorophenol and 1,3,5-trlchlorobenzene.
significant effect on the rate of photolysis of l-chloro-3iodobenzene. When the reaction rate constants measured in the presence of 1 p M sodium iodide were used, the calculated concentration-time profile agreed well with the measured profile depicted in Figure 3. Calculation of Quantum Yields. Given the emission spectrum of the irradiation source used (Figure 1)and the measured extinction coefficients of all compounds studied as a function of the wavelength, first the relative reactivity of the chemical was calculated as a function of the wavelength. As an illustration of the characteristic differences between the benzene and phenol derivatives studied, Figure 4 gives both the measured molar absorptivity and the relative reactivity of 1,3,5-trichlorobenzene and 3chlorophenol as a function of the wavelength. Using eq 7, quantum yields for photolysis of the halogenated compounds were calculated relative to 3-chlorophenol;for the calculation of Cx(lxex), a band width for X = 2 nm was used. The calculated values of @ are given in Table 11. Correspondence of Quantum Yields in Near-UV and Farther UV. From Figure 4 it is evident that, given the emission characteristics of the irradiation source used, most of the reactivity is located in the environmentally irrelevant region of X < 290 nm. For 1,3,5-trichlorobenzene, however, a significant fraction of the overall reactivity is located at wavelengths of >290 nm, making the transformation route studied environmentally relevant for this type of compound. In order to show that quantum yields were independent of the wavelength, we selected four halogenated compounds, absorbing light at wavelengths of >320 nm and irradiated these compounds in Pyrex reaction tubes instead of the quartz tubes normally used. In Table 111, a comparison is made between rates of photolysis and of quantum yields measured for both quartz and Pyrex reaction tubes. In both cases, quantum yields for direct photolysis were calculated using eq 7; in
Table 11. Experimental Quantum Yields for Photohydrolysis of All Halogenated Compounds Studied, Using 3-Chlorophenol as a Reference ( 1 1 ) compound
9
log @
1-bromo-3-fluorobenzene 3-fluorophenol 3-bromophenol 1,3-dibromobenzene 1,3-dichlorobenzene 3-iodophenol 3-chlorophenol 1-chloro-3-iodobenzene 3,5-dichlorophenol 1-chloro-3-bromobenzene 1,3,5-trichlorobenzene 1.3.5-tribromobenzene
0.171 0.155 0.151 0.125 0.120 0.096 0.090
-0.768
0.085 0.080
0.057 0.030 0.027
-0.810
-0.821 -0.902 -0,923 -1.017 -1.046 -1.071 -1.096 -1.242 -1.517 -1.563
Table 111. Comparison of Rates of Photolysis and of Quantum Yields Measured for Direct Photolysis of a Limited Number of Halogenated Compounds in Both Quartz and Pyrex Reaction Tubesn compound 3-iodophen01 1-chloro-3-iodobenzene 1,3,5-trichlorobenzene 1,3,5-tribromobenzene
kqua (min-') k,, (min-') 0.124 0.026 0.003 0.005
4.42 X 3.64 X 2.59 X 6.58 X
aqua aPyr 0.096
0.092
0.085
0.080
0.030 0.027
0.033 0.025
Denoted as qua and pyr, respectively.
the case of Pyrex tubes, the transmittance of the glass as a function of wavelength was taken into account (15). As can be seen from this table and as expected on the basis of the UV spectrum of the compounds irradiated, absolute rates of photolysis were found to be much lower when Pyrex tubes were used. However, the results given in Table I11 clearly show that, within experimental error, Environ. Sci. Technol.,Vol. 26, No. 1 1 , 1992 2119
Table IV. Correlations of the Logarithm of the Measured Quantum Yields, 'P, with a Limited Number of Molecular Descriptors equation
Table V. Calculated Quantum Yields, after Symmetry Correction, Using eq 9 for All Compounds Used in This Study"
corr coeff, r2
(a) log 9 = XIBS (b) log 9 = Xla (c) log @ = X1ar (d) log 9 = XIE, (e) log Q = XIBS + Xza + C (f) log 9 = XIBS + XpaI + C (9) log 9 = XlBS + XZE, + C (h) log 9 = XlE, + X ~ + OC (i) log 9 = XIE, + Xzul + C (j) log Q = XIBS + Xza + X&, + C (k)log 0 = XIBS + XZOI+ X&, + C
compound 1-bromo-3-fluorobenzene 3-fluorophenol 3-bromophenol 3-iodophenol 3-chlorophenol 1 -chloro-3-iodobenzene 1,3-dibromobenzene 1,3-dichlorobenzene 1-chloro-3-bromobenzeiie 3,5-dichlorophenol 1,3,5-trichlorobenzene 1,3,5-tribromobenzene
0.00
0.83 0.80 0.81
0.83 0.80
0.94 0.86 0.87
0.94 0.94 (I
quantum yields for direct photolysis are the same for both the near-UV and farther UV. Structure-Reactivity Relationships. For compounds bearing multiple halogen substituents, making the overall quantum yield equal to the sum of the contributions of each of the distinct halogen atoms present, first the measured quantum yields were symmetry corrected by dividing the quantum yield by the corresponding symmetry factor, a value of either 2 or 3. In order to derive a QSAR for the photohydrolysis of the halogenated compounds studied, the log of the measured quantum yields, as measured in quartz reaction tubes, was subsequently correlated with a number of molecular descriptors. Four descriptors were considered: (1)The bond strength (BS) of the carbon-halogen bond to be broken (16). This is the bond broken in the rate-determining step of the reaction. It is assumed that the quantum yield will decrease with increasing bond strength. To keep the QSAR straightforward, a single value for the bond strength was used for each type of halogen atom. (2) The summation of the Hammett u constants of the additional substituents to the benzene ring (17-19). All descriptor values used were obtained from Hansch and Leo (20). (3) The inductive constant of the additional substituents, up This descriptor was selected as an additional descriptor besides the Hammett u constants, because it was anticipated that electronic factors are of major importance during the rate-limiting step. uI describes the intrinsic tendency of a substituent to withdraw electrons, u1 being positive for electronegative groups as compared to hydrogen. It should be noted that uI values to a certain extent are correlated to the corresponding Hammett u constants. For the parameter set used in this study the regression coefficient, r2, for the correlation between the summed Hammet u constants and the summed uIconstants was approximately 0.81. uI values were obtained from Hansch and Leo (20). (4) The summation of the steric factors, E,, of all substituents to the aromatic ring (20). Several correlations were obtained by linear and multiple regressions with the structural descriptors given above. The value of the regression coefficient, r2,was chosen as an indication of the degree of correlation between the distinct descriptors and the logarithm of the quantum yield (17, 21). To determine whether addition of a variable significantly improved the regression equation, significance levels (F test) were calculated after every multiple regression step (17,22). The results of several correlations with BS, u, uI,and E, as molecular descriptors and combinations of these descriptors are given in Table IV. For the simple linear regression case, separately using each of the descriptors depicted above, regression coefficients in all cases were below 0.83. As can be seen from Table IV, 2120
Environ. Sci. Technol., Vol. 26, No. 11, 1992
BS
E*
log Q
80
-1.62
-0.827
125
-1.01
4.861
80
-1.71 -1.95 -1.52 -2.37 -2.32 -1.94 -2.13 -2.49 -2.91 -3.48
-0.887 -0.890
64 95 64 80
95 80 95 95 80
-0.907 -1.169 -1.292 -1.186 -1.166
-1.552 -1.831 -2.064
All descriptor values used are also given.
with multiple regression, the best correlation was obtained with a combination of BS and E,, with r2 = 0.94 and n = 12: log CP = -0.010 (*0.002)BS
+ 0.665 (*0.057)ES + 1.031 (fQ.116) (8)
Application of the F test showed that all coefficients are significantly different from 0 at the 0.1% ' significance level. Addition of either u or uI did not significantly improve the correlation. In eq 8, the values between parentheses show the calculated standard deviation for each of the descriptors and the intercept. Given the number of compounds and the number of descriptors used in eq 8, according to the Student's t distribution table given in ref 14, the 90% confidence interval for each of the descriptors used is equal to k1.83 times the standard deviation calculated. This means that the 90% confidence interval for log CP is equal to log CP = -0.010 (A0.004)BS + 0.665 (f0.105)E8 + 1.031 (h0.213) (9) From a mechanistic point of view, it may be derived from eq 9 that, for the set of closely related compounds studied, steric effects seem to play a dominant role during the rate-limiting step of photohydrolysis, as clearly shown by the significant contribution of E,. Electronic effects seem to be of minor importance since both u and uI do not significantly improve the correlation. However, it should be borne in mind that, given the fact that the compounds used are closely related, E, may partially reflect electronic effects. To a certain extent this is shown by the observation that for the parameter set used in this study the r2 value for the correlation between the u and E, was approximately 0.74. In a straightforward manner, eq 9 can be used to calculate the quantum yield of photolysis of a haloaromatic having only one halogen atom. For symmetrically substituted haloaromatics bearing only one type of halogen atoms, the contributions of all individual halogen atoms present need to be summed. Finally, since, as shown above for compounds bearing several types of halogen atoms, only dehalogenation of the halogen atom having the lowest bond strength value takes place, only this contribution needs to be taken into account in eq 9. For all compounds studied the calculated quantum yields, together with the descriptor values, used are given in Table V. In Table VI, a comparison is made between measured (log and calculated (log quantum yields using eq 9. The correspondence between the measured and calculated quantum yields is shown graphically in Figure 5.
Table VI. Overall Quantum Yields of Photohydrolysis, As Measured in Aqueous Solution (log @'ob,) and As Calculated Using eq 9 (log Ocslc), after Correction for Symmetry compound 1-bromo-3-fluorobenzene 3-fluorophenol 3-bromophenol 3-iodophenol 3-chlorophenol 1-chloro-3-iodobenzene l,3-dibromobenzene 1,3-dichlorobenzene 1-chloro-3-bromobenzene 3,5-dichlorophenol 1,3,5-trichlorobenzene 1,3,5-tribromobenzene
;j
-
log @obB -0.768 -0.810 -0.821 -1.017 -1.046 -1.071 -1.203 -1.224 -1.242 -1.397 -1.994 -2.040
log @cak -0.827 -0,861 -0.887 -0.890 -0.907 -1.169 -1.292 -1.186 -1.166 -1.552 -1.831 -2.064
A
0.059 0.051 0.066 0.127 0.139 0.098 0.089 0.037 0.076 0.155 0.163 0.023
measured extinction coefficienM as a function of the wavelength of all halogenated compounds studied in the spectral region 250-350 nm (8 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St., N.W., Washington, DC 20036. N l bibliographic citation (journal, title of article, authors' names, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $17.50 for photocopy ($19.50 foreign) or $10.00 for microfiche ($11.00 foreign), are required. Canadian residents should add 7% GST.
Registry No. 3-Iodophenol,626-02-8; 3-bromophenol,591-20-8; 3,5-dichlorophenol, 591-35-5; 3-chlorophenol, 108-43-0; 3-fluorophenol, 372-20-3; l-chloro-3-iodobenzene, 625-99-0; l-bromo-3fluorobenzene, 1073-06-9;1,3-dibromobenzene, 108-36-1; 1,3-dichlorobenzene, 541-73-1; 1,3,5-tribromobenzene, 626-39-1; 1chloro-3-bromobenzene,108-37-2; 1,3,5-trichlorobenzene, 108-70-3.
-0.9
Literature Cited
-1.1
-1.3
Log QUANTUM YIELD (obs)
Figwe 5. Correspondence between quantum yields of photohydrolysis, as measured In aqueous solutlon (log a&,) and as calculated with eq 9 (log @ c a d
Conclusions The results of this study show that, upon application of the approach depicted above, it is quite possible to derive structure-reactivity correlations for photochemical processes. Combined with the easily accessible UV spectra of a given compound and the emission characteristics of the light source applied, the correlations can be used to calculate rates of photolysis of this compound. Since the relationships developed can be used only to calculate quantum yields of photolysis of 1,3-disubstituted haloaromatics, their applicability is still limited. Also it should be noted that the QSARa derived so far only have a limited environmental relevance since some of the compounds used in this study mainly absorb light of wavelengths of 290 nm, such as PCBs. Clearly, more research is needed to enable the application of the approach given above to other groups of compounds and to other photochemical reactions, thus not only expanding knowledge regarding photochemical transformations but also enabling predictions of the composition of the mixture of products formed upon photolysis of a given compound. Acknowledgments We are pleased to acknowledge the valuable review comments made by Dr. Richard Zepp and Dr. N. Lee Wolfe, U S . Environmental Protection Agency, Environmental Research Laboratory, Athens, GA 30613.
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Supplementary Material Available Tables comprising the relative spectral energy distribution of the light source used in the spectral region 250-350 nm and the
Received for review December 3, 1991. Revised manuscript received May 18, 1992. Accepted June 18, 1992. Environ. Sci. Technol., Vol. 26, No. 11, 1992 2121
